Recently, the reduction of CO2 gas emissions has been an important issue in ironmaking processes for environmental protection, and attempts have been made to increase the content of cold iron sources (iron sources at ordinary temperature) used as iron sources in steelmaking processes. That is, attempts have been made to reduce the content of hot metal. In the manufacture of steel products, the manufacture of hot metal in a blast furnace involves the reduction and melting of iron ore, which requires considerable energy and produces enormous CO2 emissions. In contrast, cold iron sources only require the heat of melting; therefore, the use of cold iron sources in steelmaking processes reduces the energy consumption by the heat of reduction of iron ore and also reduces CO2 emissions. Examples of cold iron sources include iron and steel scrap, cold pig iron, and direct-reduced iron.
A molten-steel manufacturing process using a combination of a blast furnace and a converter, however, has a limited capacity for melting a cold iron source because the heat sources for melting the cold iron sources are the sensible heat of hot metal and the heat of combustion of carbon and silicon in the hot metal by oxidation. Recently, hot metal has also been dephosphorized as a pretreatment, which is disadvantageous for the melting of cold iron sources. An additional dephosphorization step decreases the temperature of the hot metal and also decreases the contents of carbon and silicon in the hot metal as they are oxidized during dephosphorization. Hot-metal dephosphorization is a refining step for removing phosphorus from hot metal in a steelmaking process while minimizing a decarburization reaction in advance before the hot metal is decarburized in a converter.
Accordingly, many solutions have been proposed to increase the thermal margin of hot metal during the dephosphorization or decarburization of the hot metal so that the content of cold iron sources can be increased. For example, Patent Literature 1 proposes a method in which hot metal is dephosphorized as a pretreatment while a carbon source is added to the resulting slag and is combusted by blowing an oxygen source into the slag to transfer the heat of combustion to the hot metal.
Patent Literature 2 proposes a method in which a heat transfer medium such as iron and steel scrap powder, ferroalloy powder, or quicklime powder is supplied together with oxygen gas from a top-blowing lance to hot metal in a refining vessel, for example, during the decarburization of the hot metal or the smelting reduction of iron or chromium. This method controls the secondary combustion rate in the refining vessel within the range of 10% to 55% to transfer the heat of secondary combustion to the heat transfer medium, thereby heating the hot metal with the heat transfer medium heated by the heat of secondary combustion.
Patent Literature 3 proposes a method for decarburizing hot metal in a converter using a top-blowing lance having a quintuple pipe structure having main holes serving as oxygen gas ejection holes and an auxiliary hole serving as a flux supply hole. The auxiliary hole is independent of the supply channel for the oxygen gas to be ejected from the main hole, and a fuel gas, oxygen gas, and a refining flux can be simultaneously ejected from the auxiliary hole. While jets of oxygen gas from the main holes are kept away from each other, a flame is formed at the leading end of the auxiliary hole independently of the jets of oxygen gas. The refining flux passes through the flame, which promotes the formation of slag from the refining flux.